Technical Field
The present invention relates to a driving device and an electronic timepiece.
Background Art
In a stepping motor, it is necessary for the rotor to rotate reliably at each step. Thus in driving control of a stepping motor, it is determined whether or not the rotor has rotated (rotor rotation detection). Specifically, this determination is made by detecting back EMF (back-voltage) produced by damping when, after a driving pulse has been applied to rotate the rotor, the rotor is stopped at a prescribed step angle. A correction pulse is further applied to rotate the rotor in the case where it has been determined that the rotor has not rotated.
However, “damping” of the rotor refers to transforming the kinetic energy of the rotor into Joule heat, and thus in a certain sense is a wasteful consumption of power. Techniques that accordingly reduce the speed of the rotor near a step angle in order to reduce energy loss caused by damping are known.
For example, the Japanese Patent Document Japanese Patent Application Laid-Open Publication No. H9-266697 discloses the following: “A driving pulse supplied to a step motor is subjected to chopper control such that the duty cycle of driving pulses is set to be lower in an initial stage and a final stage than in a middle stage. This makes it possible to set an effective power distribution of the driving pulses to be lower in the initial stage and the final stage and higher in the middle stage, causing the step motor to produce a torque matching the cogging torque of the step motor. The rotor is rotated at a low speed while eliminating wasteful power consumption in the initial stage and final stage of the driving pulses, which makes it possible to reduce the amount of power consumed to drive the step motor.”
However, the level of the back EMF is proportional to the rotation speed of the rotor. Thus if the speed of the rotor is reduced near the step angle as described above, the level of the back EMF will also drop near the step angle, resulting in a drop in the accuracy of rotation detection.
Meanwhile, there is a technique that uses a magnetic saturation phenomenon arising in a stator core as a method for detecting the position of a motor having a permanent magnet in the rotor. That is, if a magnetic flux produced by the permanent magnet and a magnetic flux produced by a coil are in a mutually-strengthening relationship, magnetic saturation of the core will have an increased effect, and the inductance of the coil will drop. On the other hand, if the magnetic flux produced by the permanent magnet and the magnetic flux produced by the coil are in a mutually-weakening relationship, the magnetic saturation will have a reduced effect, and the inductance of the coil will rise. As such, measuring the inductance of the coil makes it possible to estimate the rotation angle of the rotor.